1. Field of the Invention
The present invention relates to a chromatic and spherical aberration corrector for use in a charged-particle beam system and to an aberration correction method for the system. More particularly, the invention relates to a chromatic and spherical aberration corrector using multipole elements making use of superimposed electric and magnetic fields for providing simultaneous correction of chromatic and spherical aberrations and to an aberration correction method therefor.
2. Description of Related Art
In an electron beam apparatus, such as a transmission electron microscope (TEM) or a scanning electron microscope (SEM), spherical aberration and chromatic aberration are main factors deteriorating spatial resolution. Especially, an axisymmetric lens produces positive spherical aberration in essence and, therefore, it is impossible to produce a concave lens action from this lens. On the other hand, with respect to chromatic aberration, it is impossible to remove the chromatic aberration using an axisymmetric lens. Therefore, rotationally symmetric fields produced by a multipole element are used to correct these aberrations.
O. Scherzer, Optik, Vol. 2 (1947), pp. 114-132 and H. Rose, Optik, Vol. 85, No. 1 (1990), pp. 19-24 set forth a spherical aberration corrector using two stages of hexapole elements. This aberration corrector has a pair of transfer lenses, each consisting of an axisymmetric lens, and hexapole elements disposed at the nodal points of the transfer lenses at the opposite sides. The two stages of hexapole elements produce negative third-order spherical aberration. Spherical aberration in the whole system is removed by combining the hexapole elements with an objective lens.
On the other hand, JP-A-2003-203593 sets forth a chromatic aberration corrector in which electric field-type quadrupoles and magnetic field-type quadrupoles are combined. The chromatic aberration corrector has four stages of multipole elements. The first and fourth stages of multipole elements are electric field-type quadrupole elements. The second and third stages of multipole elements have electric field-type and magnetic field-type quadrupole elements. That is, these multipole elements are of a so-called superimposed electric and magnetic field type.
In this chromatic aberration corrector, chromatic aberration is corrected in the x- and y-directions independently if the optical axis is taken in the z-direction. Therefore, the correction produces a lens action which makes an electron beam diverge in one of the x- and y-directions and which converges the beam in the other. That is, so-called line focusing is achieved.
The first stage of multipole element is mounted for the line focusing. For example, where this multipole element exerts a diverging action on the electron beam in the x-direction and a converging action in the y-direction, the multipole element forms a linear electron beam extending in the x-direction in the center of the second stage of multipole element. The second stage of multiple element corrects chromatic aberration in the x-direction and, at the same time, produces a linear electron beam extending in the y-direction on the third stage of multipole element. The third stage of multipole element corrects chromatic aberration in the y-direction in the same way as the second stage of multipole element. Finally, the fourth stage of multipole element performs an operation reverse to line focusing, i.e., returns the linear electron beam to its original shape. In this case, in the center of the second stage of multipole element, the beam represents a reciprocal space image in the x-direction and a real space image in the y-direction. Conversely, in the center of the third stage of multipole element, the electron beam represents a real space image in the x-direction and a reciprocal space image in the y-direction.
In the second and third stages of multipole elements, the deflecting force exerted on the electron beam by an electric field-type quadrupole is linearly proportional to the position of the beam within the multipole elements. Because the position of the beam in the reciprocal space image corresponds to the angle of incidence of the beam impinging on the first multipole element, it can be said that the deflecting force is linearly proportional to the angle of incidence. Furthermore, a similar principle applies to the deflecting force exerted on the beam by a magnetic field-type quadrupole. Accordingly, the deflecting forces of the fields on the electron beam having a given energy can be made to cancel out each other by appropriately adjusting electric and magnetic fields produced by the electric field-type quadrupole and magnetic field-type quadrupole, respectively, in each of the second and third multipole elements. The obtained orbit is not different from a reference orbit assumed where aberrations are not taken into consideration.
On the other hand, the refractive index of an electron beam (i.e., dependence of the deflecting force on wavelength or on accelerating force) relative to an electric field is different from the refractive index of the beam relative to a magnetic field. Accordingly, where a deflecting force on the electron beam is canceled out by a combination of electric and magnetic fields, the beam does not deviate from the reference orbit. Only the refractive index relative to the electron beam varies on the orbit. Chromatic aberration can be corrected by setting a refractive index created by the magnetic and electric quadrupoles so as to cancel out the refractive index of the objective lens.
In the above-described chromatic aberration corrector, line focusing is performed to correct chromatic aberration only in one direction. Therefore, real space image and reciprocal space image of an electron beam are focused at positions different between the x- and y-directions.
That is, an appropriate electron beam obtained by the above-described chromatic aberration corrector has a limited beam diameter for irradiating only a small field of view. Therefore, the corrector can be introduced into a scanning electron microscope (SEM) or a scanning transmission electron microscope (STEM). In the case of a transmission electron microscope (TEM), a wide range on the surface of a specimen needs to be covered with a single shot of the electron beam and be irradiated with it. Therefore, it is difficult to correct chromatic and spherical aberrations for the electron beam and thus it is difficult to utilize the aforementioned corrector.
Hence, this instrument cannot be used in TEM. Furthermore, if the electron beam is made to diverge excessively in one direction by line focusing, there is the danger that the beam collides against the inner wall of the vacuum vessel. If such a collision takes place, emission and scattering of electrons produce undesired noise. In addition, the degree of vacuum may be deteriorated uselessly.